Punching, stamping rivets are used to connect two flat workpieces. The shank of the rivet is hereby pressed (punched) through the workpieces forming two punched holes. The lower workpiece, viewed in punching direction, consists of a plastically deformable material, usually a sheet metal. The shank has a shank groove near the shank end. An annular ring is stamped after the punching operation around the shank end into the lower workpiece by a press tool, thus pressing material of the lower workpiece into the shank groove.
The shank groove has in the case of a known rivet a trapezoidal shape in cross section. When workpieces of a hard material are processed, then there exists the danger that the shank will break in the area of the shank groove. When the upper workpiece consists of a soft material, as, for example, of a soft sheet metal or plastic, then material is sheared off from the upper workpiece by the upper edge of the shank groove and is pressed into the shank groove. The material-receiving space for receiving the material of the lower workpiece is thus reduced and a poor riveted joint is created. When the upper workpiece consists of plastic, then the entire shank groove may be filled with plastic so that a riveted joint is not created. The stamping rivets are manufactured as a turned part, for example on a lathe.
Two factors are, among others, of importance for the strength of the riveted joint. The first factor is that the shank groove is supposed to be filled totally and exclusively with material of the lower workpiece. The second factor is the angle at which the section (surface) of the shank groove, which section is adjacent the shank and, extends. Ideal would be here an approximation to an angle of 90.degree. which, however, would significantly increase the above-mentioned danger of breakage.